P_all_ldk.tps from A giant nektobenthic radiodont from the Burgess Shale and the significance of hurdiid carapace diversity

Radiodonts, stem-group euarthropods that evolved during the Cambrian explosion, were among the largest and most diversified lower palaeozoic predators. These animals were widespread geographically, occupying a variety of ecological niches, from benthic foragers to nektonic suspension feeders and apex predators. Here, we describe the largest Cambrian hurdiid radiodont known so far, <i>Titanokorys gainesi</i>, gen. et sp. nov., from the Burgess Shale (Marble Canyon, Kootenay National Park, British Columbia). Estimated to reach half-a-meter in length, this new species bears a very large ovoid-shaped central carapace with distinct short posterolateral processes and an anterior spine. Geometric morphometric analyses highlight the high diversity of carapace shapes in hurdiids and show that <i>Titanokorys</i> bridges a morphological gap between forms with long and short carapaces. Carapace shape, however, is prone to homoplasy and shows no consistent relationship with trophic ecology, as demonstrated by new data, including a reappraisal of the poorly known <i>Pahvantia</i>. Despite distinct carapaces, <i>Titanokorys</i> shares similar rake-like appendages for sediment-sifting with <i>Cambroraster,</i> a smaller but much more abundant sympatric hurdiid from the Burgess Shale<i>.</i> The co-occurrence of these two species on the same bedding planes highlights the potential competition for benthic resources and the high diversity of large predators sustained by Cambrian communities.

放射齿类（Radiodonts）是起源于寒武纪大爆发（Cambrian explosion）的干群真节肢动物（stem-group euarthropods），曾是古生代早期体型最为庞大、多样性最高的捕食类群之一。该类群地理分布广泛，占据了多样的生态位：从底栖觅食动物到自游滤食动物，乃至顶级捕食者。本文描述了目前已知最大的寒武纪赫德虾类放射齿动物（hurdiid radiodont）——新属新种*Titanokorys gainesi*，其化石发现于不列颠哥伦比亚省库特尼国家公园大理石峡谷的布尔吉斯页岩（Burgess Shale）。该新物种预估体长可达半米，拥有极为硕大的卵圆形中央背甲，带有清晰的短后侧突与前棘。几何形态计量学分析（geometric morphometric analyses）结果显示，赫德虾类的背甲形态具有极高多样性，且*Titanokorys*填补了长背甲类群与短背甲类群之间的形态学空白。然而，本次研究的新数据（包括对鲜为人知的*Pahvantia*的重新评估）表明，背甲形态易发生趋同演化，且与物种的营养生态位并无稳定关联。尽管背甲形态差异显著，*Titanokorys*与*Cambroraster*拥有相似的耙状附肢，可用于筛分沉积物；后者是布尔吉斯页岩中体型更小但丰度远高于前者的同域分布赫德虾类。二者在同一岩层层面的共存现象，揭示了寒武纪群落中大型捕食者间潜在的底栖资源竞争，以及当时大型捕食者类群的高度多样性。